ROLE OF PROTEIN GLYCOSYLATION IN DEVELOPMENT

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The modB mutation in the cellular slime mold Dictyostelium discoideum produces protein-linked glycans which appear to lack most of their original mass. In diploid strains homozygous for mutant alleles at this locus, it is also observed that 1) the putative contact sites A glycoprotein (gp80) is reduced to ca. 10% of its normal level, 2) reassociation of dissociated aggregation stage cells is markedly reduced, and 3) smaller fruiting bodies are formed. The expression of two other glycoproteins which normally bear the missing carbohydrate epitope do not appear to be affected at the level of resolution of immunofluorescence and SDS gel analysis. Based on these findings, we hypothesize that reduced gp80 expression on the cell surface results in reduced cell reassociation following dissociation and in reduced fruiting body size. We will investigate this hypothesis in the context of the larger question of glycan function in the accumulation and localization of developmentally-regulated glycoproteins. We will first investigate the basis for the decreased level of gp80, using synthesis and turnover studies and EM localization methods. We will then generate new mutations which alter the modB-dependent glycan, examine these modifications in terms of monosaccharide composition, glycan size distribution, and complementation analysis and then test for coordinate effects on gp80 expression, cell reassociation and fruiting body size (spore number). We speculate that by restoring a part of the missing glycan, or by removing only a part of it, by mutational means, we should be able to dissociate these effects of modB on gp80 accumulation and cell reassociation if they have a different structural basis. If we are unable to to dissociate the effects with even subtle changes in carbohydrate structure, we will infer that they are in fact due to a common mechanism (employing a common recognition process based on common structure) which prevents gp80 from arriving at the cell surface (or removes it once it arrives). Finally, glycan parameters for the other two glycoproteins carrying the carbohydrate epitope will be compared with these for gp80 to seek an explanation for the different effects of modB on their accumulation. We will compare the role of the modB-dependent glycan with another glycan, expressed on spore coat proteins, which carries an epitope recognized by another group of monoclonal antibodies. Using methods worked out for the modB-dependent glycan, we will investigate the role of this distinct glycan in distributing the spore coat proteins into prespore vesicles (whose exocytosis is developmentally regulated) and, following secretion, localizing the spore coat proteins into the spore coat, slime sheath and stalk tube.